1. Field of the Invention
The present invention relates to a generator (alternator) that can be mounted on a vehicle such as a passenger car or a truck.
2. Description of Related Art
With the progress of computerization in automobiles, many mechanical functions of the automobiles have been replaced by electric or electronic functions. Under such condition, electric power demands of the automobiles are increasing more and more, which causes vehicle generators to increase in size. Accordingly, the rotors of the vehicle generators are also increasing in size, causing inertia moments thereof to increase.
Meanwhile, the serpentine belt drive system in which auxiliaries of a vehicle including a vehicle generator are belt-driven by a vehicle engine through the same single belt is becoming widespread for adaptation to the trend of reducing the size of an engine room of a vehicle in an effort to provide a sufficiently large cabin space. Such a serpentine belt drive system has a mechanism, which is known as the auto tensioner, for absorbing belt tension variation to keep belt tension constant. Since the auto tensioner can prevent the belt tension from lowering over time, it is common that the belt tension is set at a low value in a vehicle having the serpentine belt drive system.
Incidentally, in view of fuel consumption reduction as one measure of environmental protection, there is a tendency to set an idling speed of an engine of a vehicle at a lower value, because idling of the engine does not contribute to traveling of the vehicle, and the frequency with which the engine runs at the idling speed is high. Also, in order to clean an exhaust gas of a diesel engine, use of the common rail system, which causes the pressure in combustion chambers of the engine to increase considerably, is spreading.
As a result of these factors intertwined, the rotational speed of a recent vehicle engine tends to easily fluctuate in its idle range in synchronization with the explosion cycle thereof. This idling speed fluctuation causes belt tension variation due to inertia torque variations of various belt-driven auxiliaries to increase. Especially, the inertia torque variation of a vehicle generator has a large effect on the belt tension variation, because the inertia moment and pulley ratio of the vehicle generator are large compared to other belt-driven auxiliaries. The increase of the belt tension variation causes the auto tensioner to oscillate with a large amplitude. As a result, there arise problems that the auto tensioner can interfere with other auxiliaries, a large noise is produced due to flattering or slippage of the belt, and the lives of the belt and the auto tensioner are shortened.
To cope with such problems, generators of some vehicles are provided with, in the pulleys thereof, an overrunning clutch that transmits a torque in one way (refer to Japanese Patent Examined Application Publication No. 7-72585, for example). The pulley provided with the overrunning clutch offers the following advantages.
While an engine speed is decreasing, if the rotational speed of a rotor of a vehicle generator exceeds a rotational speed of the pulley, the overrunning clutch idles, so that the rotational speed of the pulley decreases smoothly together with the engine. On the other hand, even while the engine speed is increasing, the overrunning clutch keeps idling until the rotational speed of the pulley reaches the rotational speed of the rotor. This makes it possible to prevent an inertia torque due to an inertia moment of the rotor from transmitting to the pulley, so that abrupt decrease of the belt tension on a stretched side and abrupt increase of the belt tension on a loosened side can be suppressed. As a result, it becomes possible to prevent the auto tensioner from oscillating with a large amplitude, suppress noise due to belt slippage, and prevent the belt from being damaged early on.
However, the use of the pulley provided with the overrunning clutch causes the following problem.
The rotor of a vehicle generator disconcerted from a vehicle engine by the action of the overrunning clutch keeps idling for a certain time period even after the vehicle engine is stopped. Since the rotor of the vehicle generator keeps rotating together with a cooling fan mounted thereto during this time period in a state that the vehicle engine and other auxiliaries in the same engine room are all stopped, and since the rotor generally includes Lundell type magnetic poles having a cooling fan effect, the sound of the rotating rotor, and the wind noise of the cooling fan or the magnetic poles which are harsh to hear become conspicuous during this time period.
To cope with this problem, it is known to pass a current to a field coil of a vehicle generator upon detecting an engine stop state in order to reduce the time needed for the rotor of the vehicle generator to stop, as disclosed in Japanese Patent Application Laid-open No. 2002-51593, for example).
The noise reduction technique disclosed in this patent document exerts effect on the noise caused by the rotor rotating idly when the engine is in a stopped state. However, this noise reduction technique cannot cope with the noise produced during a time period in which a vehicle transmission is upshifted several times consecutively while a gas pedal is depressed, as explained below.
In this time period, an engine speed decreases rapidly each time the vehicle transmission is upshifted, while the rotor continues to rotate idly at high speed for a while due to its inertia, although it gradually slows down. While the rotor rotates idly at high speed, since the rotation noise of the rotor, wind noises of the cooling fan and the magnetic poles, and interference noise of the cooling air interfering with the stator disposed outwardly of the rotor are high-pitched, they are easily heard harshly. It should be noted that the frequency with which the gas pedal is depressed causing the transmission to be upshifted is much larger than the frequency with which the engine is stopped.